This invention relates to a detection and location apparatus for failed fuel in a reactor utilizing tag gas.
Conventionally, there is known a failed fuel detection and location system (FFDL) in which stable tag gas with modified composition, e.g. rare gas, is previously enclosed in the respective fuel pins of a plurality of fuel assemblies disposed inside a reactor, so that the location of a broken fuel pin, if any, is detected by collecting and analyzing tag gas which is released in cover gas when such breakage is occurred.
Since the concentration of the tag gas released in cover gas in the reactor, however, is extremely low, it is impossible to determine the composition of the tag gas by directly analyzing the same, according to the existing analysis technique. Therefore, it is necessary to collect the released tag gas securely and to enrich the same to an analyzable concentration.
Conventionally, the collection and enrichment of tag gas are achieved by adsorbing the gas with low-temperature activated charcoal cooled to approximately -80.degree. C. or lower and heating it to about 350.degree. C. for removal. In "Design and Manufacture of Gas Tags for FFTF Fuel and Control Assemblies," NUCLEAR TECHNOLOGY, vol. 26, page 472, Aug., (1975) appears a method in which tag gas is collected and enriched by using activated charcoal which is cooled to -79.degree. C. Further, stated in "Proceedings of the Specialists' Meeting on In-Core Instrumentation and Failed Fuel Detection and Location," pages 313 to 317, AECL-5124, May, (1974) is a method in which tag gas is collected by means of activated charcoal cooled to -100.degree. C. and then selectively removed by reheating and sweeping with helium. However, a low-temperature activated-charcoal adsorption apparatus requires large-sized cooling means utilizing a large quantity of liquid nitrogen. Moreover, the selective desorption of the tag gas adsorbed by low-temperature activated charcoal requires extremely difficult selection and control of desorption conditions including temperature, removal time, degree of vacuum, flow rate of sweeping gas, etc., as well as complicated operations. Furthermore, in the low-temperature activated-charcoal adsorption apparatus, impurities such as water contained in various gases may coagulate at an extremely low temperature, tending to cause clog in piping.